1. Field of the Invention
The present invention relates to a method for ascertaining an offset of an output signal of an evaluation circuit integrated in a sensor, preferably of a combustion chamber pressure sensor installed in a combustion chamber of an internal combustion engine, in which the output signal is supplied to a control unit that analyzes the output signal, and a reset of the evaluation circuit for an adjustment of the output signal to a basic level is performed prior to each measuring operation of the sensor.
2. Description of the Related Art
Sheathed-element glow plugs including a pressure sensor are in use today, which measures the cylinder pressure during the combustion of a fuel-air mixture in an internal combustion engine. Such sheathed-element glow plugs including the pressure sensor have an evaluation circuit, which is connected downstream from the pressure sensor in the sheathed-element glow plug. The pressure sensor operates on the basis of a piezoelectric measuring method, in which the detected pressure is converted into an electrical output signal, which is output and detected by an engine control unit. Because of the piezoelectric measuring method, the evaluation circuit connected downstream from the pressure sensor must be reset to the zero level with the aid of an internal algorithm prior to each combustion process, in order to avoid drift of the output signal.
As a result of the input-side electrical connection of the evaluation circuit of the pressure sensor, it can happen that the output signal exhibits an abrupt change to higher or lower voltage values following an internal reset. These abrupt changes are attributable to interference signals at the input of an integrator stage of the evaluation circuit at the instant immediately after the integrator resetting.
The signal jumps following the reset result in an offset shift of the entire pressure signal, and thus also of the output signal of the pressure sensor supplied as voltage, in the following cycle.